Will it boot if set to AHCI? If so, what about in RAID with a single drive attached?
If neither boot your system you might have lost the AHCI/RAID drivers (iRST). I don't pretend to know why windows might do this. Does your system have secondary SATA ports? (If it is an intel board they won't be black.. red or blue probably).
If so there is a crazy workaround that might work. First install the drivers for the secondary port (it won't be iRST..it will be some other driver). Then move your boot drive(disconnect the mirror drive) to that port and boot off it. Now reinstall iRST.
Now swap it back to the intel ports and switch BIOS to RAID. Still boot with just one drive. Shutdown and attach the second drive. Hopefully now it will detect the second drive (that is now degraded) and rebuild it.
None of this should risk your data.. but do so at your own risk.